This popped into my head while i was checking insurance through my bank recently. Hope someone can shed some light on this for me.

Do i have to inform my bank, that i am now british? Technically when i oppeneed my bank account it was oppened my old SA passport. So they would have on their records that i was not brithsh.
Could this change affect things for me later. Could they say refuse to honour my insurance, if i ever needed to claim because of it?

Telling your employer would not make any difference - they would not give you a pay rise!

You may consider telling your bank. My wife opened an account with NATWEST many many years ago with a non-EU passport. Last few months she went to ask the bank for an overdraft, they requested to see her passport. When she gave them her British passport they said to her "but on our system it says xxx passport". They were one of those uneducated people who think that to be British you have to have been born British. They had to take days to verify the passport before increasing her bank overdraft.